Yeast killer toxins are of therapeutic and industrial use because of their effectiveness and specificity. They are used to control commercial fermentations, to identify biotypes of pathogens, and may be useful as a treatment for yeast infections. The toxins kill only specifically targeted sensitive yeast strains, yet little is known about variation among killer toxins from natural sources and almost nothing of the forces affecting that variation. This project will provide data on variation among yeasts from natural habitats and investigate some of the ecological forces responsible for maintenance of that variation. The proposed work will add t our knowledge of the exploitable varieties of toxin and provide a basis for extending the search for other useful variants. Natural microbial variation is the pool from which both resistant strains and new antibiotics appear. To accomplish these goals, the investigation will collect killer Pichia kluyveri from naturally occurring necrotic cactus tissue, one of the yeast's habitats. Killer phenotypes will be identified by screening each killer isolate for its ability to kill a set of 70 other yeasts. By collecting from geographically isolated locations and from several different host cacti, the isolates are expected to sample the range of natural variation in this trait. Once identified, phenotypes will be tested for their ability to kill 1) yeasts from the same host and location, and 2) yeasts from other hosts and locations. As each host type has a characteristic resident yeast flora that differs from the flora on other cacti, comparing the results from each test will measure the degree to which killer factors are adapted to local conditions. Local adaptation may allow for high levels of global variation. The proposed work includes experiments that test specific hypothesis linking variation among killer yeasts to variation in local conditions, both physical and biotic. Growth rates on native and non-native cactus tissue will be compared to test growth as a possible mechanism for local adaptation. Multi-species experiments will test competitive success as another possible mechanism for local adaptive success. Last, the effect of the killer yeasts on the development of species of Drosophila, the yeast's vector, will also be examined. Students will learn to identify yeasts, conduct experiments under sterile conditions, and analyse their data. The interdisciplinary aspects of the project will integrate what they learn in several courses and aid in developing quantitative and writing skills. By allowing students to commit themselves to a project for three years, research will become a natural expression of their interest in science and, consequently, an exciting career choice.